This application is in the field of multipole rod assemblies such as used in mass spectrometers and, more specifically, relates to a mass analyzing spectrometers and methods for fabricating multipole mass analyzing spectrometers. Various mass spectrometers are known in the art. An example of a prior art multipole mass spectrometer is illustrated in FIG. 1. For convenience of description, the mass spectrometer example of FIG. 1 is specific to a quadrupole mass analyzer, however embodiments of the invention may be used in other types of multipoles, for instance, hexapoles, octopoles, etc. In the mass spectrometer of FIG. 1, the sample molecules are injected by injector 105 into an ionization chamber 110, which ionizes the molecules, thereby acting as an ion source 110. Ions from the ion source 110 are focused and transferred to the mass analyzer 125 via ion guide 115, which is driven by voltage generator 120.
As shown in FIG. 1, four conductive rods, constituting the quadrupole mass analyzer 125, are arranged in two pairs, each pair receiving the same DC+RF signal, denoted as U+V*cos(w*t), wherein U is the magnitude of the DC voltage while V is the magnitude of the RF signal. One pair of rods receives a positive DC signal at zero phase, while the other receives a negative DC signal at a 180 degrees phase shift (−[U+V*cos(w*t)]), thereby acting as a band pass and separating the ions according to their mass to charge ratio, generally denoted as m/z. This relationship is illustrated in FIG. 2, wherein the shaded area denotes the band-pass wherein only ions having a mass to charge ratio (m/z) within the shaded area may pass the mass analyzer. The width of the band pass is controlled by the signal applied to the rods, such that the narrower the band pass is, the higher the resolution of the mass spectrometer.
By scanning the magnitude of U and V, one can over time allow species of different mass to charge ratio to pass through the spectrometer, thereby obtaining a spectrum of the ion species within the sample material. Generally, during the scanning the ratio UN is kept constant so as to maintain the same band pass. The ions exiting the mass analyzer 125 are detected by detector 145. As shown, controller 140 controls the power applied to the focusing optics and the mass analyzer 125.
In spectrometers, such as the mass spectrometer described above, ions of the proper m/z ratio must be kept at the center of the mass analyzer. This confinement is controlled by the electric field generated by the rods (poles) when they are energized. Therefore, the rods must be accurately manufactured and accurately positioned with respect to each other. That is, in order to maintain a proper field that confines ions to the center of the mass analyzer, a high level of symmetry must be maintained in the spatial positioning of the rods.
The high precision required in manufacturing and assembling the various parts of the mass analyzer have led to attempts aimed at achieving the precision and symmetry requirements, while reducing manufacturing tolerances and costs. The rod spacing precision that is generally aimed at during manufacturing of a typical quadrupole rod assembly is in the order of five micrometers or lower. According to some proposals the mass spectrometer is fabricated in two parts which are then mated to each other. However, such a proposal requires that the two halves be precisely machined so that after assembly they maintain symmetry among all of the rods about the ion transfer axis. According to other proposals, the rods are attached to a mandrel for alignment and then adhered to insulators. Once cured, the mandrel is removed. However, once the adhesive cures, it is rather difficult to remove the mandrel, often requiring lubricants and cooling of the mandrel to cause thermal contraction of the mandrel. This process may also damage or cause misalignment of the rods. Further information concerning the state of the art can be obtained from, for example, U.S. patent publications U.S. Pat. No. 6,926,783 and 2006/0102835.
The U.S. Pat. No. 4,990,777 to Hurst et al. discloses a pole rod assembly where metallic rods are, in a radial direction, spot welded to L-shaped brackets. The brackets are, in an axial direction, spot welded on a flat lateral face to a metallic ring which serves to provide operating voltages to a subset of rods via the intermediate brackets. The metallic ring used for distributing the operating voltages among the subset of rods is glued, likewise in an axial direction, on a flat lateral face to a ceramic holder ring.
In view of the prior art, however, there is still a need for methods for easy and cost effective fabrication of highly precise rod assemblies such as those used as mass analyzers.